This invention relates to a method and apparatus for controlling an active antenna.
Active antennas of this type are used, for example, for SAR-sensors (Synthetic Aperture Radar). They are mounted on a vehicle, such as a satellite, in such a manner that their major lobe, points perpendicularly to the flight direction, laterally several degrees away from the nadir point. The elevation angle describes the beam steering about the longitudinal axis of the antenna in a plane perpendicularly to the flight direction. The azimuth angle indicates rotations of the vehicle (flight direction, normal line of antenna).
An active antenna normally comprises a rectangular n.multidot.m matrix of individual active transmitting/receiving elements. By means of a suitable phase and amplitude distribution among the independently controlled transmitting/receiving elements, the far zone of the overall antenna can be modelled within wide limits while the geometry of the antenna remains unchanged. It is thus possible to change the direction of the major transmitting or receiving lobe electronically without changing the position of the mechanical antenna. The transmitting/receiving elements which are arranged in a line along the flight direction form a so-called "elevation group"; while the transmitting/receiving elements arranged in an array form an "azimuth group".
By means of a suitable change of the phase distribution on the transmitting/receiving elements of the individual elevation groups, beam steering of the antenna lobe can be performed in the elevation direction; correspondingly, beam steering in the azimuth direction can be accomplished by suitable change of the phase distribution on the transmitting/receiving elements of the individual azimuth groups. Variable beam characteristics are advantageous in many cases. On the one hand, different areas on the ground may be illuminated at short successive measuring intervals. On the other hand, for receiving, it is possible to build large-area antennas with a high directivity and to follow with the major lobe in the direction from which the echo of the transmitted pulse originates. The overall sensitivity of the system will increase; the microwave transmitting output can be lowered in the case of the same requirements with respect to the interval between the wanted signal and the noise signal.
In the known arrangements, a centrally generated electric microwave signal for control of the antenna is guided with the correct phase and amplitude to the individual transmitting/receiving elements of the antenna. Coaxial or wave guide components are used for this purpose. These arrangements are mechanically complicated and relatively heavy. In addition, individual phase or amplitude adjustment is possible only in the proximity of the individual transmitting/receiving elements which, in turn, limits the control possibilities.
U.S. Pat. No. 4,965,603, describes a method for controlling the transmitting/receiving elements of an active antenna in which a beat signal generated from the beaming of two lasers is guided in glass fibers to the individual transmitting/receiving elements, and is converted into an electric signal there. The frequencies of the two lasers remain unchanged. A phase modulator is used for phase distribution of the beat signal.
U.S. Pat. No. 4,583,096, discloses a method for control of transmitting/receiving elements of an active antenna in which modulated optical generators (lasers) furnish digital optical information, transported by way of optical fibers, for the control of phase actuators which are mounted directly on the individual transmitting/receiving elements. In order for the phase shifting to be exclusively a function of the digital optical information, the lengths of the optical fibers must be identical for each transmitting/receiving element.
In U.S. Patent Document 4,725,844, a method for control of transmitting/receiving elements of an active antenna is disclosed in which the light of an intensity-modulated laser is distributed to the individual transmitting/receiving units by a glass fiber network with integrated star couplers. Phase differences are generated by phase actuators coupled into the individual optical paths.
In Koepf, G. A.: "Optical Processor for Phased-Array Antenna Beam Formation;" in SPIE, Vol. 477 "Optical Technology for Microwave Applications" (1984), Pages 75-81, a laser is used to control the transmitting/receiving elements of an active antenna which emits two beams of different frequencies. In order to achieve a desired amplitude and phase distribution., the first laser beam is modulated by an electro-optical crystal, passes through a Fourier transformation lens, and is finally superposed with the second laser beam. This method is also used in German Patent Document DE 38 27 589 A1, in which, however, an additional laser is used for each antenna diagram.
Soref, R. A.: "Voltage-Controlled Optical/RF Phase Shifter," in: Journal of Lightwave Technology, Vol. LT-3, No. 5, October 1985; Pages 992-998, describes another method for controlling transmitting/receiving elements of an active antenna, in which laser light of different frequencies is coherently superposed. The desired phase distribution is generated by actuators.
It is an object of the present invention to provide a method for controlling an active antenna, by which changes of the transmitting/receiving frequency as well as the beam sweep can be accomplished in a fast and highly flexible manner. The antenna in this case comprises a number of transmitting/receiving elements which are preferably arranged in lines (elevation groups) and arrays (azimuth groups). The direction of the antenna lobes is a function of the amplitude and phase distribution of the signal on the individual transmitting/receiving elements. The above object is achieved by the method according to the invention in which, the light of a first narrow-band coherent light source is divided into n.multidot.m optical paths (n being the number of elevation groups; m being the number of azimuth groups) of different lengths. The light of a second narrow-band coherent light source--like the first light source, preferably a laser--is divided into n.multidot.m optical paths of the same or different lengths. Each of the n.multidot.m optical paths which originates from first and second light sources is assigned to one of the n.multidot.m transmitting/receiving elements (n.multidot.m channels). The totality of these optical paths forms a so-called delay network. The coherence length of the light must be sufficiently large compared to the largest running time differences of the network.
In each case, the light on an optical path which originates from the first light source, is superposed with the light on an optical path which originates from the second light source. The (optical) frequencies of the two light sources are shifted with respect to one another in such a manner that the difference frequency is equal to the frequency desired for the control of the transmitting/receiving elements. For radar applications, it is in the microwave or dwarf wave frequency range. A signal having a beat frequency equal to the difference frequency is in each case obtained from the superposition. A total of n.multidot.m beat signals of the same beat frequency are obtained.
By means of optoelectronic converters ("O/E-converters"), such as photodetectors, the n.multidot.m beat signals are converted into electrical signals. An O/E-converter detects only the beat frequency and not the optical carrier frequency of the beat signal. In the case of the optoelectronic conversion, the phase relationship of the beat signals generated by passing through the individual optical paths is maintained.
The electrical signals, which may be further amplified after conversion, are guided to the individual transmitting/receiving elements of the antenna, one transmitting/receiving element being assigned to each of them.
For a given difference in optical path lengths, the phase and the frequency (independently of one another as well as coupled with one another) of the electrical signals present at the transmitting/receiving elements may be changed by the variation of the light frequencies, so that the antenna lobes are shaped and slewed and the transmitting/receiving frequency is also changed. It is essential for the method according to the invention that no static phase shift occur (that is, a phase difference due solely to given length differences of the optical paths). Rather only a dynamic phase shift (phase shift due to a frequency change, with length differences of the optical paths being constant) must be generated.
The optical path differences are chosen such that sensitivity of the phase with respect to absolute optical path frequency changes is as high as possible. The values for the optical path differences are therefore preferably in the range of several meters to single millimeters, and thus have magnitudes which are higher than the wavelengths of the light sources.
A special advantage of the invention lies in its ability to change the phase as well as the frequency of the signals present at the transmitting/receiving elements, by changing only one quantity (one of the two light source frequencies). Thus fast and very flexible control is achieved.
Additional actuators may be arranged on the optical paths to control the phase and the amplitude, for example, by way of the polarization direction.
In addition to the general case of the control of an antenna with n elevation groups and m azimuth groups (m and n larger than 1), the method according to the invention may also be used for antennas with only one elevation group (n=1) or only one azimuth group (m=1). In either case, beam steering of the antenna lobe is possible in only one direction.
Another special case is an antenna with only one transmitting/receiving element (n=1 and m=1). Although in such case a lobe beam steering is no longer possible, the transmitting and receiving frequency (beat signal) can be changed.
In an advantageous embodiment, the light is guided in optical fibers. The light originating from the light sources is fed into optical fiber networks and is branched into the individual optical paths by means of optical couplers, such as 2.multidot.2 couplers and/or star couplers. In yet another embodiment, the light is guided by way of open distances (free space configurations) with the distribution into the individual optical paths taking place, for example, by means of partially reflecting mirror, diffraction grids, or holograms.
To generate the beat signal, sufficiently narrow-band, controllable light sources must be available. Advantageously, laser-diode-pumped Nd:YAG-lasers may be used. In the case of an optical frequency of 2.8.multidot.10.sup.14 Hz, their optical resonaters permit, by means of a control, line widths of below 1 Hz.
Because of thermal fluctuations in the millikelvin range, the line position changes rapidly. For example, the line position of a commercially available laser changes approximately 1 MHz per minute, due to the finite precision of the thermal control. Therefore a continuous outside control of the laser frequencies is required.
Although the approach using two separate lasers lead to good overall power efficiency, a single laser setup would lead to a simpler setup. G. A. Koepf (see the above mentioned SPIE proceeding) uses in his experimental optical processor one LASER. Its light is divided into two bundles. The first bundle contains light at the carrier frequency (the input light of the laser). The light of the second bundle is frequency shifted. The resulting beat note signal is controlled by the reference oscillator which drives the accusto-optic deflector. The nature of the driving signal is arbitrary, therefore it may contain modulated information on it.
With such a side band modulating setup the requirements to phase stability of the light source are low because--within the length of coherence--phase variations cancel themselves when the light interferes after a retarding network at the output.
The frequency shifting device--here the accusto-optic deflector--must feed only the light of one sideband of the modulated carrier into the second bundle. If light from the mirrored sideband also was fed into the system, the output beat note signals would show unwanted amplitude and phase variations.
It is the goal of a SAR-antenna to repeatedly transmit short centrally generated microwave pulses and to receive back the reflected echo. During the transmitting pulse, the frequency is varied linearly with time over a certain band width (chirp signals). By means of the method according to the invention, such a chirp signal can be generated purely optically without the previously required high-cost electronic construction. For this purpose, the frequency of one or both light sources is changed correspondingly, whereby the desired change of the beat frequency (transmitting frequency) is achieved.
The method according to the invention is not limited, however, to radar applications. By means of an external modulation of one of the two lasers, arbitrary information may also be transmitted by means of an antenna controlled according to the method of the invention.
The method can be used for transmitting as well as for receiving: For transmitting, the signal, which is present at the individual transmitting/receiving elements with a defined phase and amplitude, is transmitted directly after an electric amplification. For receiving, it is used as an LO-reference (local oscillator) in the transmitting/receiving elements for the coherent phase comparison with the received signal in mixer stages.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.